Electric initiator



Feb. 28, 1967 v. J. MENICHELLI ETAL 3,30

ELECTRIC INITIATOR Filed Dec. 2, 1964 mvzsmogs V/ncen/ J. Men/chemUnited States Patent Ofifice 3,306,202 ELECTRIC INITIATOR Vincent J.Menichelli and Irving Kabik, Silver Spring, Md., assignors to the UnitedStates of America as represented by the Secretary of the Navy Filed Dec.2, 1964, Ser. No. 415,547 2 Claims. (Cl. 102-28) The invention describedherein may be manufactured and used by or for the Government of theUnited States of America for governmental purposes without the paymentof any royalties thereon on therefor.

The present invention relates to electric initiators for explosives, andmore particularly, to an improved electric initiator for explosiveswhich is relatively insensitive to spurious signals and radiatedelectrical fields.

The purpose of the present invention is to provide an electricallyinitiated timer, detonator, squib, or igniter which is insensitive tothe application of electrical energy at powers below a predeterminedlevel which is above the level usually required to fire such devices.Requirements of the equipment in which the present initiator wasintended to be used dictated that the device be completely safe upon theapplication of currents below 1 ampere or 1 watt of electrical power,whichever happens to be greater, and will always function when currentsof 5 amperes or a power of 5 watts of electrical energy and above areapplied, whichever is smaller. These limits are illustrative only andare not to be read in a limiting sense. As will be more apparent fromthe detailed description to follow, the limits may be varied by changingthe functional relationships between certain parts of the device. Inaddition to the above, the desired result had to be accomplished withoutusing external heat dissipating electrical media such as the bypass orshunt circuits sometimes employed in the prior art.

Conventional methods of overcoming the problem of premature ignition ofelectrical initiators have not been satisfactory. Often these systemsare complicated and expensive, and in many instances safety is achievedat the expense of reliability. Early electrical initiators of thepresent character utilized a resistance heating element in combinationwith a charge of heat sensitive explosive material such as mercurfulminate, lead azide or diazodinitrophenol which in turn initiated amain charge. Initiators of this type are highly susceptible toinadvertent initiation, and many attempts have been made to render themsafe. One of the more important solutions which has found wideacceptance in the art involves the use of an insensitive high explosivecharge, with a so-called exploding bridgewire as the initiator. Thebridgewire will carry relatively high currents which are inadvertentlyapplied to the device but when a high voltage sharp pulse of electricalenergy is applied, the wire explodes and ignites the charge. Thissolution is ordinarily satisfactory, but would not meet the requirementsset forth above. Often it is found that inadvertently applied lowvoltages have heated the bridgewire to its fusing temperature withoutinitiating the explosive, but still rendering the device inoperativefrom that time on. This is, of course, only one example of devices knownto the prior art, but is can be said that no device has been availableheretofore which can perform in the required manner as outlined above.

Accordingly, it is an object of the present invention to provide anelectrically initiating explosive device which will operate as set forthabove and in which conventional explosives are employed.

Another object of the invention is to provide an electrical initiator inwhich the rate of transmission of heat to a heat sensitive explosive canbe more accurately controlled than has been possible heretofore.

3,3062% Patented Feb. 28, 1967 The device according to the inventionutilizes conventional explosives, of the heat sensitive type for eX-ample, but with specific structure of the electrical elements to providethe desired safety. In particular, the heater element is made of a fiatstrip of alloy. Lead-in conductors are provided, which contact theheater strip and act as heat sinks. No external heat dissipating mediumsare employed.

Other objects and advantages of the invention will occur to thoseskilled in the art from a reading of the following detailed descriptionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a cross-sectional view through the device of the invention;

FIG. 2 is a section on line 2-2 of FIG. 1; and

FIG. 3 is a partial section on line 33 of FIG. 1.

Referring now to FIG. 1, the electric initiator according to theinvention is there shown as comprising a housing or receptacle 1!} ofconventional type having a closed end 11 and an open end 12. A maincharge 13 is contained within the lower portion of the receptacle 10.

The electrically actuated means for initiating the explosion in maincharge 13 operates generally in the conventional manner by heating aheat sensitive explosive material by means of an electrical resistanceelement. The structure of the heating system is different from prior artstructures, however, in ways which will largely be obvious from thefollowing description.

A heat sensitive initiator charge 14 of conventional material such aslead azide, for example, is supported in contiguous relationship to maincharge 13 in a cup-shaped central bore 17 and an upwardly extendingcylindrical wall 18. An insulating disc 19 is provided to insureelectrical insulation between charge holder 16 and the electrical partsof the device.

The resistance heating element of the invented device is indicated at22. As shown, this resistance element is a fiat rectangular strip ofsuitable metal. In the preferred embodiment, strip 22 is of-cupronalloy, although it will be understood that other suitable materials maybe employed as desired. In order to supply electrical energy toresistance heating element 22, a pair of lead-in condoctors 23 and 24are provided. These elements contact resistance element 22 at theirlower ends. In this manner, heat is transmitted from resistance heatingelement 22 to the lead-in conductors 23 and 24 so that the lead-inconductors themselves act as heat sinks. Completing the assembly is abody 26 of glass or other material which serves to support theconductors 23 and 24. A metallic sleeve 20 surrounds body 26, permittingthe parts to be joined together and sealed by a solder joint 21. Body 26may also be of material capable of conducting heat away from strip 22,but this is not essential to the invention. The presence of conductors23 and 24 as heat sinks, provides the desired safety in the device atlow values of input current. At such low values, the resistance element22 will heat up somewhat, but the heat will be conducted away throughthe lead-in conductors so that the heat sensitive charge 14 is notheated to its detonation point. By making the resistance element flat,the relation between the conduction between the resistance element andthe lead-in conductors and between the resistance element and theexplosive is variable as hereinafter described. Upon the application ofa higher value of current or wattage, the relationship of the lead-inconductors and the heating element is such that the lead-in conductorscannot carry away sufiicient heat to prevent actuation of the lead azideinitiator charge. Under these conditions, of course, the devicedetonates.

As stated above, the use of a flat ribbon or strip as the heatingelement makes it possible to vary the relationship between the heat lossfrom the strip to the heat sinks and the heat loss to the explosive. Theamount of heat lost to the heat sinks is a function of thecrosssectional area of the strip, since heat cannot leave the centralportion of the strip except by conduction through the area of the stripwhich is elfectively in contact with the heat sinks and the effectivecontact area is the crosssectional area. The loss to the explosive is afunction of the amount of surface area of the strip in contact with theexplosive. Thus, for a given cross-sectional area and length of strip,the loss to the heat sinks is constant, and the loss to the explosivecan be varied by varying the shape of the cross-section, by varying thethickness and width of the strip, to place more or less area in contactwith the explosive as desired. It should also be noted that the rate ofheating of strip 22 is a function of the cross-sectional area. Itfollows that the rate of heating of the strip is fixed for a givencross-sectional area and the relative rate of heating of the explosivecan be varied by varying the shape of the strip. By the simple expedientof using a ribbon or strip instead of the conventional cylindrical wire,a high degree of control over the current sensitivity and response timeof the device is achieved.

Thus, a device has been described which uses conventional explosives andgenerally conventional plug structure but which is safe in that it willnot initiate upon the application of relatively low current or power butwill initiate reliably upon the application of higher values of currentor power. This is accomplished without using external heat dissipatingelectrical media.

Obviously certain modifications and variations of the present inventionwill occur to those skilled in the art in the light of the aboveteachings. It is therefore to be understood within the scope of theappended claims the invention may be practiced otherwise than as hereinspecifically described.

What is claimed is:

1. An electric initiator for explosives which is insensitive to theapplication of electrical currents or electrical power below apredetermined level which is above the level usually required to firesuch devices and which will reliably function when electrical currentsor electrical power above another predetermined level is applied,comprising a metallic receptacle having a closed end and an open end,

a main charge of explosive in said receptacle adjacent said closed end,

a cup-shaped metallic charge holder having its base in contiguousrelation to said main charge and its sidewall contacting saidreceptacle, said charge holder having a bore through its base,

a heat sensitive explosive charge disposed in said bore and in contactwith said main charge,

an insulating disk in the bottom of said charge holder,

said disk having a bore therethrough in axial alignment with the bore insaid holder,

a resistance heating element spanning the bore in said disk and incontact with said heat sensitive explosive charge, said elementcomprising a flat strip of conductive metal,

a pair of lead-in conductors in contact with said resistance heatingelement and extending out of said open end of said receptacle forconnection to a source of electric power, said lead-in conductors havinga predetermined heat capacity and acting as heat sinks,

a block of insulating material in which said conductors are embedded,and

a metallic sleeve surrounding said block of insulating material andsoldered to the inside wall of said receptacle,

whereby for low values of applied electrical current or electrical powerheat will be conducted away from said resistance heating element throughsaid lead-in conductors so that said heat sensitive explosive charge isnot prematurely heated to its detonation point.

2. An electric initiator as recited in claim 1 wherein said resistanceheating element is made of cupron alloy.

References Cited by the Examiner UNITED STATES PATENTS 2,754,757 7/1956MacLeod 102-28 2,798,921 7/ 1957 Haas 102-28 X 2,880,671 4/1959 Lutz etal 102-28 2,887,054- 5/1959 Bryan 102-28 3,059,576 10/1962 Haefner102-28 3,117,519 1/1964 Hamilton et al 102-28 3,135,200 6/1964 Jackson102-28 3,227,083 1/1966 Moses et al. 102-28 FOREIGN PATENTS 581,3168/1959 Canada.

BENJAMIN A. BORCHELT, Primary Examiner.

R. V. LOTTMANN, V. R. PENDEGRASS,

Assistant Examiners.

1. AN ELECTRIC INITIATOR FOR EXPLOSIVES WHICH IS INSENSITIVE TO THEAPPLICATION OF ELECTRICAL CURRENTS OR ELECTRICAL POWER BELOW APREDETERMINED LEVEL WHICH IS ABOVE THE LEVEL USUALLY REQUIRED TO FIRESUCH DEVICES AND WHICH WILL RELIABLY FUNCTION WHEN ELECTRICAL CURRENTSOR ELECTRICAL POWER ABOVE ANOTHER PREDETERMINED LEVEL IS APPLIED,COMPRISING A METALLIC RECEPTACLE HAVING A CLOSED END AND AN OPEN END, AMAIN CHARGE OF EXPLOSIVE IN SAID RECEPTACLE ADJACENT SAID CLOSED END, ACUP-SHAPED METALLIC CHARGE HOLDER HAVING ITS BASE IN CONTIGUOUS RELATIONTO SAID MAIN CHARGE AND ITS SIDEWALL CONTACTING SAID RECEPTACLE, SAIDCHARGE HOLDER HAVING A BORE THROUGH ITS BASE, A HEAT SENSITIVE EXPLOSIVECHARGE DISPOSED IN SAID BORE AND IN CONTACT WITH SAID MAIN CHARGE, ANINSULATING DISK IN THE BOTTOM OF SAID CHARGE HOLDER, SAID DISK HAVING ABORE THERETHROUGH IN AXIAL ALIGNMENT WITH THE BORE IN SAID HOLDER, ARESISTANCE HEATING ELEMENT SPANNING THE BORE IN SAID DISK, AND INCONTACT WITH SAID HEAT SENSITIVE EXPLOSIVE CHARGE, SAID ELEMENTCOMPRISING A FLAT STRIP OF CONDUCTIVE METAL,